Szymon Grzesiak

The global need to reduce resource consumption in the construction sector requires nonstandard solutions. One solution for achieving lightness in concrete is carbon reinforced concrete, where the carbon reinforcement enables the reduction of concrete cover, as it can not corrode. Therefore, structures that are very thin-walled can be built. However, the challenge to include a reinforcement that provides resistance against bending and shear remains. This paper presents two types of 3D carbon reinforcement. The hollow core beams are reinforced with a 3D net lattice girders and a 3D reinforcement from post-shaping a biaxial grids. To evaluate the position of the reinforcement in the thin-walled webs and whether the concrete cover is enough, a nondestructive (NDT) method, such as computed tomography (CT), is used. The novelty of this study relates to the size of the sample, where real-scale beams are scanned with X-ray under load. With the help of the CT the crack initiation could be found to be independent from the chosen carbon reinforcement and therefore the thinwebs and flanges can not be held accountable for preliminary failure. The resulting voxel data sequences is analyzed with digital volume correlation in order to detect cracks and to investigate the failure behavior. In addition, digital image correlation and inductive displacement transducers are used to verify the results. The comparison of the cracks in the lower flange of the beam shows that they can be detected by both methods and the difference in the determined crack widths is explained by artifacts in the CT scan. Improvements for further scans are given.

This paper presents a pipeline for investigating crack development and strain changes in concrete beams that are reinforced with a glass fiber-reinforced polymer (GFRP) rebar using 4D image data. Concrete beam samples were loaded and imaged simultaneously using computed tomography at several load stages. The displacement fields were estimated from the resulting time series of images using the MorphFlow method. Individual cracks were segmented and analysed at selected stages of their formation. The method shows high potential for early detection of very thin cracks at the sub-voxel level. Additionally, the adopted image processing approach provides a detailed distribution of crack widths. The crack widths measured in the experiment are consistent with numerical results and with standards specific to FRP reinforcement, such as the Canadian ISIS standard. Image analysis results also indicate that the local crack width at the rebar level differs significantly from the crack width at the sample surface.

The large-scale CT system GULLIVER was recently put into operation at the Rheinland-Pfälzische Technische Universität (RPTU) in Kaiserslautern (Germany). The project, which was launched in 2019 as a result of a competition between various German universities and research institutions, aims to use Computed Tomography (CT) to examine real-scale building components under the most realistic load conditions possible, as is customary in materials testing. The CT measurement is intended to provide new insights into the fatigue dynamics inside these concrete structures, which are up to 6 m long and 1 m wide, by detecting minimal cracks of up to 100 microns in size at the point of load application. The special design of the system and the components installed, open up completely new fields of application for high-energy CT that were previously not available. Particularly noteworthy is the system's gantry-type manipulation unit, whose use has so far been limited primarily to medical diagnostics, as well as the X-ray components used, which open up completely new fields of application for CT in terms of energy and resolution. But also, the processing of the resulting huge data of approx. 10,000 × 10,000 × 2,000 voxels generated in the system represents a particular challenge for the image processing and evaluation to be carried out following the measurement. In this publication, we present the project funded by the German Research Foundation (DFG) and its development containing the measurement results achieved during the current completion of the system as well as the status of the processing procedures developed. In addition, the future measurement campaigns planned with the system are presented and the possibilities to access the innovative testing system for own research activities are explained.

Carbon-reinforced concrete (CRC) is increasingly utilized in construction, due to its unique properties, such as corrosion resistance, high-tensile strength, and durability. Understanding its behavior under different loads is crucial to ensuring its safe and effective use in various construction applications. In this study, three-point bending tests were performed in combination with large-scale in situ computed tomography (CT). This paper presents the related three- and four-dimensional evaluation methods, with emphasis on crack width and quality control. The focus was on large CRC beams, with cross-sectional sizes of up to 80 mm by 160 mm. Such dimensions require extremely high energy during a CT scan. Therefore, a new experimental setup with energies of up to 8 MeV was used in this study. However, such high energies posed new challenges to the analysis methods. Therefore, two methods (digital volume correlation and grayscale profile analysis) for accurate crack width estimation were adapted and applied to the 3D reconstructions. In addition, a photogrammetric stereo image sequence was acquired and analyzed, using digital image correlation to cross-validate the results derived from the 3D crack width estimates. The 3D CT images also played a key role in the quality control measures, including the localization of the carbon-reinforcement and the assessment of porosity within the concrete structure.

The highly irregular crack pattern of reinforced concrete has been studied primarily at the surface. The ability to extend image correlation into the interior of structures by using X-ray computed tomography (CT) opens up new possibilities for analyzing the internal mechanics of concrete. In order to enable a complete material characterization, it is necessary to study the crack geometry at the micro level in 3D images over time, i.e., 4D data. This paper presents the results of in situ CT tests that were carried out on carbon-reinforced concrete (CRC) beams subjected to bending load. The main objective of the tests was the experimental analysis of the evolution of individual cracks at different stages of their formation by applying digital volume correlation (DVC) to the 4D image data from the computed tomography. The results obtained from the CT were compared with other measurement techniques, such as distributed fiber optic sensing, clip gauges, and digital image correlation (DIC).

Computed tomography combined with mechanical tests offers completely new insight into the behavior of concrete samples under stress. Particularly the development of new fiber reinforcement materials for concrete elements requires appropriate material models and thus for investigating the interior of the concrete structure. In 3D image data obtained by computed tomography, local structural changes within the sample due to mechanical loading can be observed without further altering the sample. We applied this state-of-the-art approach to a concrete core with an embedded glass fiber reinforced polymer rebar under increasing forces applied to pull out the rebar. In this paper, authors describe a novel in-situ setup for non-destructively 3D imaging during the pull-out test. Conducting the pull-out test leads to the formation of local pore volume changes along the rebar. These pore volume changes are not only visualized but quantified analytically based on the images. Interpreting these volume changes, we derive a novel method for calculating strain and normal stresses in the rebar. Our new method captures the detailed distribution of the bond stresses between rebar and concrete and consequently describes the bond behavior more accurate. It turns out that the observed bond behavior cannot be explained completely by commonly assumed material laws. This emphasizes the need for further, more extensive research combining 3D imaging, mechanical testing, and quantitative image analysis.

Precast concrete sandwich panels (PCSPs) are known for their good thermal, acoustic and structural properties. Severe environmental demands can be met by PCSPs due to their use of highly thermally insulating materials and non-metallic connectors. One of the main issues limiting the wider use of sandwich walls in construction is their unknown fire resistance. Furthermore, the actual behaviour of connectors and insulation in fire in terms of their mechanical performance and their impact on fire spread and the fire resistance of walls is not fully understood. This paper presents an experimental investigation on the structural and thermal behaviour of PCSPs with mineral-wool insulation and glass-fiber-reinforced polymeric bar connectors coupling two concrete wythes. Three full-size walls were tested following the REI certification test procedure for fire walls under fire and vertical eccentric and post-fire mechanical impact load. The three test configurations were adopted for the assessment of the connectors’ fire behaviour and its impact on the general fire resistance of the walls. All the specimens met the REI 120-M criteria. The connectors did not contribute to the fire’s spread and the integrity of the walls was maintained throughout the testing time. This was also confirmed in the most unfavourable test configuration, in which some of the connectors in the inner area of the wall were significantly damaged, and yet the structural connection of the concrete wythes was maintained. The walls experienced heavy heat-induced thermal bowing. The significant contribution of connectors to the stiffness of the wall during fire was observed and discussed.